• Paul C mod for Princetons

My impression of this mod is it makes the amp sound "bigger." It seems to increase the fullness of the bass. It doesn't add more bass, it just sounds different. I find I can lower the volume knob from 5 to around 4, not because it's louder but it seems to cut through the mix.

The 2M2 resistor is hooked to the B+ supply to supply the bias voltage. If you do the Stokes mod, the point shown above is on a different tap of the power supply than if the amp were stock. I tried both taps. The voltages I measured are:

My preference was for the phase inverter tap. The amp stayed cleaner a bit longer when the grid was biased with 97 V vs. 79 V--the sound of a higher bias voltage at a volume of 7 was like the sound at a volume of 5 with a lower bias voltage.I thought the distortion also had a better character to it.

I read something about the cathode voltage of a 12AX7 shouldn't be above 100V, so I'm not sure whether the tube is being stressed. I'm going to keep it this way and see if the tube fails and will post something here if it does.